When the engine is cold I'm hearing a screeching noise from under the hood

audi
a4

#1

It started happening about 6 months ago. The first thing in the morning there would be a noise from under the hood that sounded like a bad bearing. It’s not tied to the RPM of the engine so I suspect its probably an electric motor. After 30-60 seconds the sound stops and doesn’t happen again until the engine is cold again.

My first thought is that it might be a vacuum pump, possibly for the door locks, or something. Does this sound reasonable? What other type of electric motor would be under the hood? It doesn’t seem to be tied to anything else (wipers, radio, headlights, etc.)


#2

Probably not an electric motor. More likely the serpentine belt is slipping at startup. The alternator is working very hard to power all the stuff at startup AND recharge the battery from the cranking. Might be time to change the belt as well as maybe the belt tensioner is getting weak, One of the idler pulleys may also be failing although those noises usually stay as long as the engine is running.

To prove its the belt, remove it and start the car for less than a minute run time. Is it quiet on startup? Shut it off and check the tensioner and idler pulleys for proper function and look to see that the pulleys don’t have a shiny glaze on them. Replace what’s needed, steel wool the pulleys (if shiny and install a new belt. I’d bet that fixes it.


#3

There’s an emission control component under the hood called the secondary air pump.

This is an electric fan that pumps air into the exhaust stream to light off the catalytic converter.

When these wear out they can make all kinds of noises.

So, that might be what you’re hearing.

Tester


#4

I’m fairly certain it’s not a belt as I can rev the engine and the sound does not change at all. In fact, the noise doesn’t seem to start until I back the car out of the garage. This leads me to believe it’s a


#5

I think you might be on to something here. The noise sounds exactly like bearings going out in a fan or pump. I did a bit of research on this and it looks like these tend to fail at the rivets that hold the two halves together. No one has mentioned hearing bearing noise.

I’ll check on my secondary air pump and update this thread if/when I find something. Hopefully it might help the next person with the same problem. :slight_smile:


#6

To find out if the noise is from the secondary air pump, when the engine is cold unplug the electrical connector to the pump.

Now start the engine cold and if the noise is no longer there, it’s the pump.

Tester